THE OPEN UNIVERSITY GEOLOGICAL SOCIETY

WESSEX BRANCH

FIELD TRIP

SUNDAY, 16 JULY 2017

1030 hrs ,

LEADER: JEREMY CRANMER

Note: The field trip starts at Povington Hill viewing area SY 888811 (not )

Directions to Povington Hill from Directions to Povington Hill from Dorchester Wareham

At Dorchester leave A35 (turn right) on to At Wareham take A352 (direction Wool and A352 (direction Wool and Wareham). Wareham) to Wool. At Wool (level crossing) turn right on to At Wool (level crossing) turn left on to B3071 B3071 At army camp turn Left on to B3070 At (near entrance to ) turn Right onto u/c road. After 350 metres turn right at range entrance with flagpole.(GR 860817) After approx 4 Km (you are now on top of the ) turn right (GR SY 888812) into the viewing area/car park.

The Tyneham car park has an honesty box for charging

Toilets are available 100 metres east of the Tyneham car park

The fieldtrip area is covered by BGS Sheet No 329 – Bournemouth and OS Landranger45 Sheet 195 – Bournemouth, Purbeck and surrounding area

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Welcome to this beautiful part of the Dorset coast which in any weather shows Dorset geology and scenery at its finest. If you haven’t been here before you will learn a lot about the geological setting of Dorset from about 140 Ma years ago to the more recent past. If this is a revisit, you are sure to add to your knowledge and perspective.

SAFETY

This locality is freely accessible to the public and is enjoyed by thousands of visitors every year, but as with any coastal exposure of cliffs and rocks, there are numerous hazards to be aware of and precautions that can minimise the risk of injury that could curtail your enjoyment of your visit. Specifically: -

Sea Cliffs are liable to rock falls at any time, avoid standing beneath overhangs and generally keep away from the base of the cliffs. Wear a hard hat at all times on the beach, it will protect your head from small rock falls.

Rock hopping over pebbles or boulders presents trip or slip hazards, avoid wet surfaces particularly if covered by sea weed.

Within the army ranges there is a SPECIAL HAZARD from live shells. Obey all notices that denote restricted access to the range walks and DO NOT TOUCH any ordinance you may come across. Do not dig or hammer in the cliffs or beach for the same reason. Hammers are banned from Mupe and Worbarrow bays.

Generally the use of hammers in the is discouraged, but there are ample specimens of all rock types on the foreshore that can be reduced in size with care and consideration for other people. You are advised to wear goggles when doing so.

High Tide = 1240

BASIC ITINERARY

Povington Hill (SY 888811). Following a brief introduction we shall consider the geological structure to the south and west where we can see the Jurassic beds forming the Weymouth Anticline. Looking north we can see the Tertiary formation of the River Frome Syncline lying in the Wessex Basin

We shall the go by car for about 2km to Tyneham Village Car Park (SY 882802). (Honesty Box = £2.00 per car) Toilets available. Tyneham village was commandeered by the in 1940 to provide ranges and a training area. The inhabitants were given short notice (48 hours?) to leave but understood that they could return after the war. They`re still waiting!

We shall then walk for about 1km to Pondfield Cove (SY 871796), stopping briefly at Worbarrow on the way to discuss the overall geology. You will need to bring your lunch with you. Pondfield Cove shows the lying conformably beneath the Purbeck beds. You should find examples of • Faulting

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• Fossils • Trace fossils, • Breccia • Ripple marks, • Halite pseudomorphs.

Ma at STAGES Dorset outcrop Max Thickness at start outcrop 74 Mastrichtian Not exposed Not exposed 83 Campanian Portsdown Fm White Culver Fm Upper Chalk 260m 86 Cretaceous Santonian Newhaven Fm Group Seaford Fm Lewes Fm 88 Coniacian 90 Turonian New Pit Fm 41m ______Holywell Fm 97 Cenomanian Grey Zigzag Fm 57m Chalk West Melbury Fm Group Upper 60m 112 Albian Gault 12m Lower Lower 60m Cretaceous Greensand 125 Aptian 132 “Wealden” Wealden Shales 716m Wealden beds 146 Ryazanian Purbeck Beds 119m 152 Upper Portlandian Portland Stone 73m Jurassic Portland Sand 154 Clay 532m

We shall then have lunch and walk on the shingle westwards along the beach at Worbarrow Bay. In doing so we are walking forward through time starting from the Purbeck formation which was laid down in coastal and agonal conditions at about 40oN. The change from shallow seas (Portland) to tidal lagoons (Purbeck) is explained by a marine regression.

At the end of the Ryazanian, i.e. Purbeck beds, the was a conmsiderable climatic change, from arid conditions to one of high rainfall. This change is marked by the end of the evaporic environment to one of increase clay minerals. The kaolinite content of the clay minerals increases with acidity This increase in acidity is the resultof increased raionfall. Nevertheless the effects of this acidity must have been long term and tempered because there still much evidence of carbonates in the deposits.-

This regression continued and resulted in dry land which extensive terrestrial flora and fauna. These are the Wealden Beds which are well displayed at Worbarrow Bay albeit in a very compressed form. You will be able to identify • River deposits • Lake deposits, • Fossil vegetation • Crossbedding

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• Diagenic changes resulting from ground water • Changes in lithology owing changes in environmental factors.

However the terrestrial conditions didn`t last beyond the Aptian when a marine transgression commenced. By the Albian the Gault Clay (deposited in moderately deep, tranquil and probable anoxic environment) and The Lower Greensand (deposited in relatively shallow, high-energy, well oxygenated seas) had been deposited and this is visible at the western end of the beach. Continuing deepening of the seas resulted in the deposit of the Grey Chalk of the Zigzag Fm (W Melbury Fm is missing here) and ultimately the white chalks of the Holywell, New Pit and Lewes Fms capped by the Portsdown Fm at Cow Corner (SY 861804). Unfortunately the upper reaches of the chalk are inaccessible here.

We shall then return to Worbarrow (near ). If you have sufficient energy and time I recommend walk up to Tyneham Cap (SY890798). The coastal views are stunning. There are also the well displayed which forms the anticline important in trapping the oil for the “nodding donkey” near the beach. Pleased keep within the yellow range markers.

The village of Tyneham (next to the car park) has been preserved as it was when the inhabitants left in 1940. It is well worth a visit too.

Further information Dr Ian West`s website at http://www.southampton.ac.uk/~imw/worbar.htm

References: House M R: “Geology of the Dorset Coast” Mortimore R M.: “Logging the Chalk” Cretaceous gems of the Jurassic Heritage Coast of Rory N. Mortimore* OU SXR260: “Geological History of the British Isles” Raggett, George: OUGS (Wessex) trip handout West, Ian: His website http://www.southampton.ac.uk/~imw

Jeremy Cranmer March 2017

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Fig.77. Boulders on the beach at Worbarrow Bay, Dorset, illustrate the Albian Upper Greensand (Bindon Sandstone Member of Gallois and Owen, 2017) and Cenomanian Grey Chalk contact (at this locality). The top of the Upper Greensand is a the very tough limestone (Worbarrow Bay Hardground) with a green-glauconite mineralised top surface (A) and an extensive burrow network below with burrow walls also mineralised with green glauconite (A and C). The base of the Cenomanian Grey Chalk is represented by the Basement Bed, overlain by the Glauconitic Marl (B). The top Albian limestone hardground passes down into a richly glauconitic, burrow-mottled greensand (D, E and F) with numerous bivalve fossils, especially oysters (labelled ‘I’ in photo E). E and F basal surface of the massive hardground shown in A to D, a conspicuously burrowed glauconitic sand (see Fig.12 for field sections at Wobarrow Bay, and ).

Cretaceous gems of the Jurassic Heritage Coast of England Rory N. Mortimore*

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